Semiconductor evolution continues to advance toward development of smaller arid smaller devices to provide faster operating transistors and higher density chips. However, this march toward smaller dimensions requires more sophisticated processes and equipment. For example, newer, yet mature, higher resolution and higher energy lithography techniques, such as E-Beam lithography, are being refined and developed to allow fabrication at the nanometer level rather than the micrometer level. As the lithography tools create finer and finer resolution for smaller dimension patterning, imperfection in a water substrate becomes a critical aspect of chip fabrication quality control. If the surface of a substrate is warped or wavy, the projected image from the lithography tools will become distorted and have out-of-specification image dimensions. Hence, systems are needed to more accurately detect wafer imperfections and to support modifications to accommodate those imperfections, thereby maintaining and improving production yields.
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, integrated circuits and other semiconductor systems, including devices requiring submicron dimensions. A frequently used substrate for such applications is a semiconductor wafer. While this description is written in terms of a semiconductor wafer for illustrative purposes, one skilled in the art would recognize that this description also applies to other types of substrates and lithographic processes known to those skilled in the art.
Various lithography systems have differing types of exposure apparatus which may be used depending on the application. For example, x-ray, ion, electron or photon lithography each may require a different exposure apparatus, as is known to those skilled in the art.
In the lithography process, accurately determining the dimensions and surface topography of the target substrate is critical to maximizing resolution of the projected image. Accurate height measurement is essential to allow proper focus of the beam used to pattern sub-micron features. Two key factors affecting a user's ability to accurately measure substrate height are substrate transparency and substrate flatness.
With non-transparent substrates, and at coarser resolutions, height may be more easily measured because the light from a height measurement device, such as a red laser, is reflected from the surface of the non-transparent substrate back to the height detector sensor. Thus, to a limited degree, the height of the non-transparent substrate can be measured somewhat accurately, and, adjusted to be within an optimal focal range for pattern writing on the substrate by the lithography tool. Contrarily, this height detection method cannot be used for transparent substrate for two reasons. First, when used on a transparent substrate, the reflected laser signal needed for height detection is so small that it cannot be detected accurately by the sensor. Second, the substrate non-flatness must be leveled to the focus latitude of the lithography tool's capabilities.
Typically, an aluminum film is applied on top of resist on a transparent substrate used in E-Beam lithography to allow sufficient reflection to detect the substrate height. This method suffers from two deficiencies. First, depending on the thickness of the aluminum film, the laser beam may be partially transmitted through the aluminum film, the layer of resist, and finally into and through the transparent substrate. Consequently, a number of different reflective signals are produced which cannot be accurately interpreted by the height detection sensor to provide a true and accurate height measurement. Additionally, since aluminum film is coated on top of the resist, the actual height of the surface of the substrate is not being measured, the aluminum film will have differing heights, and hence, additional variations are introduced which reduce ability to obtain a measurement to place the substrate in the desired focal range.
Alternatively, a reflective film or metal, such as gold, may be deposited on the underside of a transparent wafer to provide reflection of a specific wavelength. Again, the accuracy of this method still hinges on the thickness of metal deposited on the underside of the wafer, the thickness of the wafer, and, the particular wavelength of the laser. These multiple variables coalesce to create inherent inaccuracies in height detection using this approach.
Further, certain broadband wavelengths can be used to enhance height detection measurement accuracy, but once again, there remains inherent variability in the measurement.
An apparatus and method is needed that will allow a non-flat and transparent wafer substrate to be positioned in a substrate holder such that the substrate is placed in an optimal focal range for the desired purpose. In particular, there is a need for such a method and apparatus to support next generation lithography tools to create smaller and smaller images on substrates, including transparent substrates.